ABSTRACT: Hermansky-Pudlak syndrome (HPS) is a rare form of pulmonary fibrosis that results from mutations in genes that regulate the function of lysosomes and lysosomal-related organelles. Although mutations in the alveolar epithelium have been linked to the development of HPS lung fibrosis, the mechanisms by which these mutations enhance susceptibility of the lung to injury are unknown. In recent work, we uncovered a novel mechanism by which epithelial-specific HPS mutations promote fibrotic remodeling in the lung. We have found that deficiency in the AP3 protein (a known HPS gene) leads to marked reduction in expression of p53 in the alveolar epithelium by increasing ubiquitin-proteasome degradation of this protein. Further, we uncovered that expression of p53 is essential for regulating antioxidant and barrier defenses in the lung epithelium and we also found that strategies which increase p53 expression can effectively restore epithelial homeostasis and reduce fibrotic remodeling in the lungs of bleomycin-exposed AP3 deficient mice. Based on these observations, we propose the following central hypothesis regarding the pathogenesis of pulmonary fibrosis in HPS: We hypothesize that AP3 deficiency enhances susceptibility to pulmonary fibrosis by decreasing p53 expression in the distal lung epithelium, and strategies that augment p53 expression can both attenuate lung injury and reduce fibrotic remodeling to bleomycin in the HPS mouse lung. This central hypothesis will be tested in 3 Specific Aims. In Specific Aim 1, we will test the hypothesis that AP3 deficiency reduces p53 expression by enhancing ubiquitin-proteasome degradation. We will determine whether genetic and pharmacological approaches that block the degradation of p53 enhance antioxidant and epithelial barrier defenses and reduce fibrotic remodeling in the bleomycin-exposed lung. In Specific Aim 2, we will confirm that expression of p53 is critically important in maintaining antioxidant and barrier defenses in the lung by performing various in vitro and in vivo loss- and gain-of-function studies in wild-type and AP3 deficient injured and uninjured tissues. Lastly, in Specific Aim 3, we will test the hypothesis that decreased macroautophagy in the alveolar epithelium of AP3 deficient mice contributes to accumulation of the ubiquitin ligases which target p53 for degradation. We will further test whether strategies augmenting macroautophagy can reduce p53 degradation, augment anti-oxidant and epithelial barrier defenses and ameliorate bleomycin-induced fibrotic remodeling in the lung. In summary, this proposal will establish the mechanisms by which AP3 deficiency enhances susceptibility to pulmonary injury/fibrosis and lay the foundation for future clinical investigations testing whether therapeutic strategies targeting these mechanisms can attenuate the onset or progression of pulmonary fibrosis in HPS.